The electric power steering controller disclosed in a patent document, JP 2013-52793 A (patent document 1) estimates a road surface load from a steer torque and an assist torque instruction value, and sets a target steer torque from the estimated road surface load. More specifically, based on the sum of the steer torque and the assist torque instruction value, a total steer torque that is applied to the mechanical element of the steering system is estimated. The total steer torque tends to become greater as the road surface load increases, thereby enabling a setting of the target steer torque based on an assumption that the total steer torque is equal to the road surface load.
Further, the motor is driven based on the assist torque instruction value that is calculated from a deviation of the steer torque from the target steer torque, so that the steer torque matches (e.g., is equated to) the target steer torque by the assist torque generated by the motor. According to the above-described configuration of the patent document 1, the target steer torque is controlled to have a value according to the road surface load, thereby providing, for the driver, a steering feel that accords with the road surface load when he/she operates a steering wheel.
Further, in the electric power steering controller disclosed in another patent document, JP 2004-299492 A (Patent document 2), for example, the target steer torque is generated based on the steer angle, and the electric power steering controller controls the assist torque output from the motor to match the target steer torque.
An increase of the frictional force of the mechanical elements of the steering system, which are controlled objects of the electric power steering controller, leads to an increase of a required torque for a driver-desired steering operation.
In the technique of the patent document 1, the road surface load is estimated as an addition of the frictional force of the elements of the steering system to the actual road surface load. Thus, when the frictional force of the elements increases, the estimated load has to have a greater value. In other words, since the estimated load is used to generate the target steer torque, an increase of the estimated load under the influence of the frictional force of the elements of the steering system leads to an increase of both of the target steer torque and the assist torque instruction value.
Under the influence of the frictional force (i.e., friction), when the steering wheel is steered toward one of the extreme positions to the right or to the left (i.e., a steer-away situation), the target steer torque at the time of such steering operation has to be increased, thereby necessitating an increase of the steer torque to have a large value.
In a steering wheel return situation, in which the steering wheel is steered back to a neutral position (i.e., a steer-back situation), under the influence of the friction, the steer torque decreases to 0 before the steer angle actually returns to 0 degree.
In other words, a returnability of the steering wheel, (i.e., ability for the steering angle to return to 0 degree), is poor under the influence of the friction, thereby necessitating the driver to “force” the steering wheel to return to the neutral position.
In particular, on a small friction road surface where a friction between the tire and the road surface is very small (i.e., on a low μ road), the influence on the steering operation, on the friction of the mechanical elements of the steering system increases relative to (i.e., in comparison to) the travel on a high μ road.
Therefore, on the low μ road, the steering toque in the steer-away situation is under a relatively-large influence of the friction of the elements of the steering system. In addition, when the influence of the friction is large, the returnability of the steer angle is greatly deteriorated.
Further, a self-aligning torque at the time of a low speed travel decreases in comparison to a high speed travel time, which also results in an increase of the friction influence of the steering system, thereby deteriorating the returnability of the steer angle.
On the other hand, in the electric power steering controller of the patent document 2, the target steer torque is determined according to the steer angle, and the target steer torque is not affected (i.e., changed) by the change of the friction of the mechanical elements of the steering system.
Therefore, the change of the friction of the mechanical element of the steering system does not affect the steering feel of the driver at the time of steering the steer wheel to a certain steer angle of a predetermined value.
However, the target steer torque in the patent document 2 is not changed according to the change of the road surface load. Therefore, such a configuration does not allow the driver to feel the change of the road surface load when he or she operates the steering wheel.
For example, it makes no difference for the driver in terms of steering feel whether he or she is traveling on the high μ road or on the low μ road. In other words, it is impossible for the driver to sense (i) the road surface condition (e.g., a friction coefficient between the tire and the road surface) or (ii) a grip of a steered tire on the road etc., based on the steering feel of the steering wheel, thereby deteriorating the operability of the steering system than one in the patent document 1.